1. Brief Introduction of ISR (Idle State Signalling Reduction) Function
ISR is a technology of reducing idle state signalling. After a UE (User Equipment) is registered with both an MME (Mobility Management Entity) and a SGSN (Serving GPRS Support Node, where GPRS stands for General Packet Radio Service) and ISR is activated, the UE in an idle state moves between a list of registered TAIs (Tracking Area Identities) of an E-UTRAN (Evolution-Universal Terrestrial Radio Access Network) covered by the MME and a registered RA (Routing Area) of a UTRAN (Universal Terrestrial Radio Access Network) covered by the SGSN without any signalling interaction with the network. ISR is activated under the control of the network, the MME/SGSN sets whether to activate ISR in a TAU/RAU Accept (TAU stands for Tracking Area Update, and RAU stands for Routing Area Update) message, and the UE activates or deactivates ISR in response to the indicator.
2. Brief Introduction of CSFB (Circuit Switched Fallback or Simply CSFB or CS Fallback) Function
CSFB supports characteristics of a CS (Circuit Switch) service of an LTE (Long Term Evolution) user equipment over a traditional 2G/3G network. The CS service characteristics include a CS call, an SMS (Short Messaging Service), an MT-LCS (Mobile Terminating Location Services), an MO-LCS (Mobile Originating Location services), etc.
The LTE UE is attached to EPS (Evolved Packet System) services and non-EPS services via a Combined EPS/IMSI Attach process in an LTE system. Upon successful attachment, a SGs interface is set up between a Mobility Management Entity (MME) of the EPS services and a mobility management entity MSC/VLR (Mobile Switching Center/Visited Location Register) of the non-EPS services, and the UE resides in an E-UTRAN. When there is a CS call coming to the MSC/VLR, if the UE is in an idle state, the MSC/VLR pages the UE through the MME, and upon reception of paging, the UE firstly performs an SR (Service Request) process and then falls back to a 2G/3G system to answer the call.
The terms “MSC/VLR” and “VLR” in the invention can be used interchangeably and have the same meaning.
3. Brief Introduction of SMSonly (Short Messaging Service-Only Characteristic)
SMSonly refers to the characteristic of SMSonly introduced in a 3GPP LTE network because a network operator requires that no CSFB be deployed but an SMS be supported in its network in view of a demand for directing an LTE UE in need of CS voice (voice centric) to have an access over a traditional 2G/3G network while having a UE in need of data (data centric) reside in an LTE network and offering an SMS over the LTE network.
A UE in need of SMSonly is attached to EPS services and non-EPS services via a Combined EPS/IMSI Attach process in an LTE system. In this Combined EPS/IMSI Attach process, the UE shall transmit an indicator of SMSonly to the network to indicate its need of the EPS services and an SMS for use. If SMSonly is supported over the network, the support of SMSonly over the network is indicated in an Attach Accept message. Upon successful attachment, a SGs interface is set up between a Mobility Management Entity (MME) of the EPS services and a mobility management entity MSC/VLR of the non-EPS services, and the UE resides in an E-UTRAN. When there is an MT-SMS (Mobile Terminating-SMS) coming to the MSC/VLR, if the UE is in an idle state, the MSC/VLR pages the UE through the MME, and upon reception of paging, the UE firstly sets up a signalling connection with the network and then can receive the MT-SMS from the MSC/VLR via an NAS (Non Access Stratum) signalling message. In this case, the UE will not fell back to a 2G/3G network.
Reference can be made to 3GPP 23.272 for a description of CSFB and SMSoverSGs (“SMS over SGs interface”). Reference can also be made to 3GPP 23.272 (updated with the document S2-097374) for a relationship between CSFB and/or SMSonly and ISR. In this application document SMSonly and SMSoverSGs have the same meaning and can be used interchangeably.
A drawback of the prior art lies in the following problem.
If a VLR setting up a SGs interface with an MME is a default VLR, there is a possible absence of a signalling connection between the VLR and a radio access network node of a current GERAN/UTRAN of a UE, so that even if the UE responded to paging, the response message might have been discarded by the network, thus resulting in wasted network resources occurring at an MME-SGSN interface, at a GERAN/UTRAN air interface and at an MSC/VLR. Also an unnecessary process may be incurred at the UE.